Membrane formation in the chamber angle after failure of argon laser trabeculoplasty: analysis of risk factors

Aim-Membrane formation in the chamber angle induced by argon laser trabecul oplasty (ALT) can be a cause of treatment failure. Identification of risk f actors for membrane formation was the primary aim of this retrospective stu dy. Methods-Semithin sections of trabeculectomy specimens obtained in a 2 year period were examined by light microscopy. 122 eyes which were treated with one or more ALTs before trabeculectomy were identified. In 46 eyes, a suffi cient amount of trabecular meshwork was obtained to permit morphological an alysis. Results-Eyes treated with ALT had a significantly higher incidence of membr ane formation (p=0.001). In 23/46 specimens a cellular and collagenous memb rane was observed covering the entire trabecular meshwork. In 14/23 specime ns (61%), this membrane was readily visible at low power magnification (x40 ). Comparison of these eyes with those without membrane formation revealed a significant difference in the number of ALTs (mean 2.07 (SD 0.73) v 1.48 (0.59);p=0.026) and in preoperative IOP (32.0 (9.7) v 26.2 (8.4) mm Hg; p=0 .04). Conclusions-Membrane formation in the chamber angle is a frequent cause of ALT failure. The major risk factor is the number of ALTs performed.